A Fabry-Perot interferometric device (FPID) can be configured to transmit electromagnetic waves (e.g., light) of a predetermined wavelength. Generally, FPIDs include an optical cavity—often referred to as a Fabry-Perot cavity—that is formed between two reflectors (e.g., mirrors) in the FPID. Some FPIDs are configured so that the gap between the two reflectors can be altered by moving either or both of the mirrors using, for example, a micro-electrical-mechanical system (MEMS). Varying the gap facilitates precisely tuning an FPID to a particular wavelength. Since visible light colors are distinguished by wavelength, a tunable FPID may therefore be controllably configured to transmit different colors of visible light. Additionally, a tunable FPID may be configured to not transmit light. Therefore, a tunable FPID may operate, for example, as a red/green/blue/black (RGBB) device.
Referring to Prior Art FIG. 1, an FPID 100 includes two parallel members 110 and 120 positioned a distance d1 apart in an orientation that creates an FP cavity. Reflective layers on members 110 and 120 make these members operate as reflectors. When an incident light enters FPID 100 at an angle α, a stationary standing wave pattern is produced between parallel members 110 and 120. When the FP cavity has a width that is an integral number of half wavelengths, light beams having a specific wavelength with a resonant range are output.
To select desired wavelengths for output (e.g., red, green, blue), an FPID may have stops that facilitate controlling the locations to which a moveable member (e.g., 120) may be moved. These stops may facilitate precisely controlling the location of the moveable member and thus may facilitate precisely controlling the width of the optical gap. However, conventional FPIDs may experience stiction problems due to charge trapping that occurs at or near stops.
The FP cavity in FPID 100 may initially be set to a first desired wavelength λ1 by orienting members 110 and 120 parallel to each other at a distance d1. The FP cavity in FPID 100 may then be set to a second desired wavelength λ4 by orienting members 110 and 120 parallel to each other at a distance d4. Distance d1 and distance d4 may be associated with stops. Additionally, members 110 and 120 may be positioned at locations between stops associated with d1 and d4 to form a gap having widths corresponding to d2 and d3. Some example FP cavity gap sizes may include:
Gap SizeColor1000 ABlack2000 ABlue12500 AGreen13000 ARed13800 ABlue24800 AGreen2
In some FPIDs, the moveable member may be repositioned using an electrostatic actuator. However, as the moveable member is repositioned, an electric field may build at and/or near the stops and/or at and/or near different components that may come in contact with each other. This electric field may lead to charge trapping. Additionally, if various FPID components come in contact, they may short out. Clearly this is undesirable. Thus, to prevent shorting, stops may be fabricated from a dielectric material. While fabricating stops from a dielectric material may reduce shorting, this fabrication technique may increase charge trapping. Increased charge trapping may have negative consequences that include actuation signal screening and stiction.
In some devices that include multiple FPIDs, CMOS circuitry is provided on a substrate in an array corresponding to the placement desired for the multiple FPIDs. A structure(s) may then be fabricated above the CMOS circuitry. The structure(s) may include, for example, a fixed top plate, a moveable middle plate, and a fixed bottom capacitor plate. The moveable middle plate may be a reflective pixel plate that is supported by flexures attached to the substrate. The structures and circuitry include opposite plates of a capacitor. Applying a charge or voltage between the opposite plates facilitates attracting and/or repelling the middle plate by electrostatic forces. In the micron and submicron sizes associated with MEMS FPIDs, voltages of a few volts compatible with CMOS circuitry can create a suitable displacement (e.g., 500 Å). The positions to which the middle plate may be moved can be controlled by stops.